The present invention relates to methods for protecting semiconductor copper bond pad surfaces with ceramic coatings that are sufficiently frangible during ball, wedge or flip chip bonding to obtain metal-to-metal contact between the bonding surfaces and the wires bonded thereto. The method protects the copper bond pads during extended exposure to water and water solutions such as are experienced during sawing.
The use of copper bond pads on semiconductor devices would be an attractive alternative to that of aluminum, were it not for atmospheric contamination of the copper surface, which oxidizes readily to form a coating that is not removable by standard methods of wire bonding machines, and requires the use of fluxes in solder-type interconnects, e.g., flip chip bonding. Present attempts to overcome this problem involve the use of a cover gas that is unavoidably expensive and complex and restricts bond head and work holder movement, or the use of a noble metal or overplating with inert metals which are more costly and can lead to the formation of unwanted intermetallic compounds at the bond pad interface.
U.S. Pat. No. 5,771,157 encapsulates a wedge bond of an aluminum wire to a copper pad with the resin, after the bond is formed. No protection against oxidation is provided to the copper pad prior to wedge bonding.
U.S. Pat. No. 5,785,236 protects a copper bond pad from oxidation with a surface layer of aluminum. This detracts from the advantages sought to be obtained by replacing aluminum bond pads with copper bond pads.
There remains a need for methods by which copper bond pad surfaces may be protected from oxidation prior to wire bonding or flip chip soldering.
This need is met by the present invention. Ceramic coatings have now been developed for the bonding surfaces of copper bond pads that are sufficiently frangible to obtain metal-to-metal contact between the bonding surface and the wire bonded thereto during ball or wedge wire bonding, and to obtain a surface suitable for soldering without fluxing.
It has also been discovered that the same ceramic coatings can be generally used to protect the copper surfaces of electronic packages. That is, the present invention provides ceramic coatings for the protection of the copper surfaces of organic substrate packages, metal substrate packages ceramic substrate packages, and the like.
According to one aspect of the present invention, a method is provided for protecting the surface of an uninsulated portion of a copper circuit from environmental contamination detrimental to joining the surface to another metal surface, wherein the method includes the step of coating the surface with a layer of a ceramic material having a thickness that is suitable for soldering without fluxing and that is sufficiently frangible when the surfaces are being joined to obtain metal-to-metal contact between the surfaces.
The invention is particularly suited to protecting the bonding surfaces of copper bond pads. Therefore, in a preferred embodiment of the present invention, the uninsulated portion of the copper circuit is the bonding surface of a copper semiconductor bond pad.
The present invention thus provides electronic packages having uninsulated copper circuit surfaces with coating layers that are capable of being removed at bonding or soldering. Therefore, according to another aspect of the present invention, an electronic package is provided containing at least one uninsulated copper surface coated with a layer of a ceramic material having a thickness that is suitable for soldering without fluxing and which provides the layer with the aforementioned hardness. In a preferred embodiment, the electronic package is a semiconductor with uninsulated copper bond pads.
This aspect of the present invention includes electronic packages having uninsulated portions of copper circuits coated with a layer of rare earth metals that form complexes with copper. The layer has a thickness that, upon formation of the copper complex and exposure to a reducing environment containing hydrogen, forms a ceramic hydride layer having a thickness that is suitable for soldering without fluxing and which provides the layer with the aforementioned hardness.
This aspect of the present invention thus also includes electronic packages having uninsulated portions of copper circuits with protective ceramic metal hydride coatings. Therefore, according to another aspect of the present invention, an electronic package is provided containing an uninsulated portion of a copper circuit coated with a surface layer of a metal hydride compound selected from metal hydrides of copper-rare earth metal complexes and metal hydrides of copper-immiscible metals that form metal hydrides, in which the surface layer has a thickness that is suitable for soldering without fluxing and which provides the layer with the aforementioned hardness. Again the preferred electronic package is a semiconductor having at least one copper bond pad.
The inventive method provides the ability to bond wires to copper circuits using existing equipment without modification of the wire bonder, and without additional costs and limitations of cover gas technology and hardware. The foregoing and other objects, features, and advantages of the present invention are more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawing.